Source : Webster's Revised Unabridged Dictionary (1913)

Knee \Knee\, n. [OE. kne, cneo, As. cne['o], cne['o]w; akin to
   OS. knio, kneo, OFries. kn[=i], G. & D. knie, OHG. chniu,
   chneo, Icel. kn?, Sw. kn["a],Dan. kn[ae], Goth. kniu, L.
   genu, Gr. ?, Skr. j[=a]nu, ? Cf. {Genuflection}.]
   1. In man, the joint in the middle part of the leg.

   2. (Anat.)
      (a) The joint, or region of the joint, between the thigh
          and leg.
      (b) In the horse and allied animals, the carpal joint,
          corresponding to the wrist in man.

   3. (Mech. & Shipbuilding) A piece of timber or metal formed
      with an angle somewhat in the shape of the human knee when
      bent.

   4. A bending of the knee, as in respect or courtesy.

            Give them title, knee, and approbation. --Shak.

   {Knee breeches}. See under {Breeches}.

   {Knee holly}, {Knee holm} (Bot.), butcher's broom.

   {Knee jerk} (Physiol.) a jerk or kick produced by a blow or
      sudden strain upon the patellar tendon of the knee, which
      causes a sudden contraction of the quadriceps muscle; one
      of the so-called tendon reflexes.

   {Knee joint}. See in the Vocabulary.

   {Knee timber}, timber with knees or angles in it.

   {Knee tribute}, or {Knee worship}, tribute paid by kneeling;
      worship by genuflection. [Obs.] ``Knee tribute yet
      unpaid.'' --Milton.

Knee jerk \Knee jerk\ (Physiol.)
   A jerk or kick produced by a blow or sudden strain upon the
   patellar tendon of the knee, which causes a sudden
   contraction of the quadriceps muscle.

Source : WordNet®

knee jerk
     n : a reflex extension of the leg resulting from a sharp tap on
         the patellar tendon [syn: {knee-jerk reflex}, {patellar
         reflex}]